text: How the labeling of RNA molecules is achieved depends on the model organism. For E.coli (bacteria) the minimum medium M9 can be used and supplemented with the stable isotope labeled variants of the needed salts. This enables labeling with C-carbon, N-nitrogen, S-sulfur and H-hydrogen. In S.cerevisiae (yeast) there are currently two possibilities: First, the use of commercially available complete growth medium, which enables labeling with C-carbon and/or N-nitrogen and second the use of minimal YNB medium which has to be supplemented with several amino acids and glucose which can be added as stable isotope labeled variants in order to achieve C-carbon, N-nitrogen and H-hydrogen labeling of RNA.
  While labeling in model organisms like E.coli and S.cerevisiae is fairly simple, stable isotope labeling in cell culture is much more challenging as the composition of the growth media is much more complex. Neither the supplementation of stable isotope labeled glucose nor the supplementation of stable isotope labeled variants of simple precursors of nucleoside biosynthesis such as glutamine and/or aspartate is sufficient for a defined mass increase higher than 2 Da. Instead, most cells kept in cell culture can be fed with stable isotope labeled methionine for labeling of methyl groups and with stable isotope labeled variants of adenin and uridine for labeling of the nucleoside's base body. Special care must be taken when supplementing the medium with FBS (fetal bovine serum), as it also contains small metabolites used for the biosynthesis of nucleosides. The use of dialyzed FBS is therefore advisable when defined labeling of all nucleosides is desired.